THURSDAY, Dec. 5 (HealthDay News) -- Although measles has been virtually eliminated in the United States, outbreaks still occur here. And they're usually triggered by people infected abroad, in countries where widespread vaccination doesn't exist, federal health officials said Thursday.

And while it's been 50 years since the introduction of the measles vaccine, the highly infectious and potentially fatal respiratory disease still poses a global threat. Every day some 430 children around the world die of measles. In 2011, there were an estimated 158,000 deaths, according to the U.S. Centers for Disease Control and Prevention.

"Measles is probably the single most infectious of all infectious diseases," CDC director Dr. Thomas Frieden said during an afternoon news conference.

Dramatic progress has been made in eliminating measles, but much more needs to be done, Frieden noted. "We are not anywhere near the finish line," he said.

In a new study in the Dec. 5 issue of the journal JAMA Pediatrics, CDC researcher Dr. Mark Papania and colleagues found that the elimination of measles in the United States that was announced in 2000 had been sustained through 2011. Elimination means no continuous disease transmission for more than 12 months.

"But elimination is not eradication. As long as there is measles anywhere in the world there is a threat of measles anywhere else in the world," Frieden said. "We have seen an increasing number of cases in recent years coming from a wide variety of countries. Over [this] year, we have had 52 separate, known importations, with about half of them coming from Europe."

Before the U.S. vaccination program started in 1963, an estimated 450 to 500 people died in the United States from measles each year; 48,000 were hospitalized; 7,000 had seizures; and some 1,000 people suffered permanent brain damage or deafness.

Since widespread vaccination, there has been an average of 60 cases a year, Dr. Alan Hinman, director for programs at the Center for Vaccine Equity of the Task Force for Global Health, said at the news conference.

But, Frieden pointed out, "We have seen a spike this year with 175 cases and counting. Nine outbreaks, including three large ones -- New York City, North Carolina and Texas, and 20 hospitalized cases."

All of the U.S. outbreaks were tied to people who brought back measles from overseas. Most of those sickened weren't vaccinated, Frieden added.

Speaking at the press conference, Hinman said: "It's nice to be worrying about 175 cases. It's a mark of progress, but it also shows how much further we have to go. Measles is so infectious that before a vaccine was available essentially every child in the United States had measles before the age of 15. That means every year, on average, there were 4 million cases."

Dr. Paul Offit, chief of the division of infectious diseases and director of the Vaccine Education Center at Children's Hospital of Philadelphia, said: "Because we don't see much measles, and we haven't seen measles deaths in this country for years, that doesn't mean it's not just right around the corner.

"People think measles is not a big deal and they're wrong," he added. "Not only have we largely eliminated measles, we have eliminated the memory of measles, and so we don't realize how sick measles can make you."

Hinman said he was concerned about parents who don't have their children vaccinated for religious or other reasons. "Particularly clusters of people who reject vaccinations, which leads to localized outbreaks when measles is imported into the United States," he said.

Like smallpox, measles can be eliminated, but only if the vast majority of a population is vaccinated. Since 2001, the CDC and other agencies have vaccinated 1.1 billion children around the world. These efforts have prevented 10 million deaths -- one-fifth of all deaths prevented by modern medicine, according to the CDC.

Since measles vaccination began 50 years ago, at least 30 million children worldwide have survived who otherwise would have died from the disease, Frieden said.

Around the world, however, measles still takes an enormous toll in lives, said Dr. Peter Strebel, who's with the World Health Organization.

"Despite progress, measles remains a formidable enemy," he said, citing recent large outbreaks in Nigeria, Pakistan, Spain and the United Kingdom.

Many countries lack the resources to combat the problem, Strebel said. And according to the CDC, only one in five countries can quickly detect, respond to or prevent health threats caused by emerging infections.

Strengthening surveillance and lab systems, training disease detectives and increasing the ability to investigate disease outbreaks would make the world -- and the United States -- safer, the CDC said.